And she said that “on behalf of the British people” she thanked “our friends and allies around the world” who have made it clear that they stand with the United Kingdom at this time.

Police have searched six addresses and made eight arrests in Birmingham and London, Mrs May said.

The attacker’s identity is known to the police and MI5, she said, and would be made public when possible.

He is British born - and “some years ago” was investigated by MI5. But he was “a peripheral figure”, Mrs May said.

It follows Wednesday’s terror attack in which four people, including the attacker, died.

Victims included police officer PC Keith Palmer, a 48-year-old husband and father with 15 years of police service; a woman in her mid 40s, and a man in his mid 50s.

Aysha Frade was the first member of the public to be identified as a victim, with her principal at DLD College London describing her as “highly regarded and loved”.

Prime Minister Theresa May has spoken about the London attack in the House of Commons

Seven people are in a critical condition, and 29 have been treated in hospital.

The attacker, armed with two large knives, jumped out of the car after smashing it into the railings encircling the Palace of Westminster, fatally stabbing PC Palmer as he entered the grounds. He was shot dead moments later by another officer.

A group of French schoolchildren were among those targeted on the bridge, with three injured

MPs went back to work, determined to return to business as usual in the wake of the terror attack on Parliament.

A minute’s silence was held nationwide at 9.33am, including in the Palace of Westminster and at New Scotland Yard, to commemorate the three innocent people killed in Wednesday’s assault on Westminster.

The time for the silence was chosen in honour of Keith Palmer’s shoulder number - 933.

Mark Rowley, the Met’s senior anti-terror officer, said six addresses were raided overnight in London, Birmingham and elsewhere.

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Twenty-nine people were treated in hospital and seven remained in a critical condition on Thursday following the attack, he added.

Neighbours have described the dramatic moment armed police stormed a Birmingham flat linked to the terror attack.

Stunned residents watched as more than a dozen black-clad officers equipped with machine guns smashed their way in to an address in Hagley Road, Edgbaston, Birmingham, at about 11pm on Wednesday.